/**
 * Calculates a relative em value from $px relatively to $px-context.
 * @param  {px value} $px         Value.
 * @param  {px value} $px-context Context.
 * @return {em value}             Relative em value.
 */
@function px-to-em($px, $px-context) {
  @return $px / $px-context * 1em;
}